stand: use a common function in gfx_fb for setting up teken colors

These are basically identical, with exception to the hook installed
which is specific to the loader we're building by necessity. Pull these
out into common/gfx_fb.c and just parameterize the hooks to make it
easier to change the logic.

Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D50886